1. Field of the Invention
This invention resides in the field of laboratory equipment used in performing assays on a multitude of species or reaction media simultaneously in individual wells of a microtiter plate and multi-well plates in general that are designed for performing large numbers of small-volume assays simultaneously. In particular, this invention addresses matters arising when assays in these plates are read or monitored by optical scanning.
2. Description of the Prior Art
The standard microtiter plate with 96 wells in a 12×8 array and a spacing of 9 mm between wells is one example of the wide variety of multi-well plates used in performing automated chemical or biochemical assays for purposes such as screening or determining binding affinities, reactivities, or other characteristics of large numbers of species. Other multi-well plates contain as few as 24 wells or as many as 1536 wells, with various sizes in between. The species analyzed in these plates are often biological species such as proteins or nucleic acid fragments, but can also be small molecule libraries randomly prepared by chemical laboratories for purposes such as cheminformatics and bioinformatics, or more specifically, chemical or biological activity screening, gene function determination, and target validation.
Optical scanning is widely used for detection in these plates, since optical scanning can be performed by scanning heads that perform all of the detection functions and yet are small enough to focus on individual wells while rapidly traversing the entire well array. In addition, optical data from multiple wells is readily stored, quantified, and otherwise processed by automated instrumentation. To achieve high performance with large numbers of small-sized wells, the most effective optical scanning systems are those that utilize confocal optics with a depth of field that is on the order of a few millimeters or less. Since the height of the typical multi-well plate far exceeds this range, many multi-well plates are constructed with flat bottoms of glass or other transparent material so that scanning can be performed through the bottom of the plate. This is particularly effective when the solvents and other suspending media have been removed from the wells and the reaction products are deposited in a layer on the floor of each well. Accurate and uniform scanning however still requires that the plate be held in a secure position at a uniform and controlled distance from the travel plane of the scanning head. With the small distance, typically from about 1.0 mm to about 1.7 mm, that must be maintained between the surface of the glass and the scanning head, the plate cannot be grasped or secured from the rear, i.e., the scanning head side, since any such securement would interfere with the travel of the scanning head and any specialized fixtures that might be built into the plate bottom for this purpose would either do likewise or require that the plate be increased in length or width to accommodate such fixtures. Irregularities in the thickness of the transparent bottom will further interfere with the scanning accuracy.